As nathan says, you have to have some kind of knowledge of materials and manufacturing processes. I would add human factors/ergonomics and basic marketing skills to list as well if you expect to be taken seriously by employers looking for Industrial Designers. A basic knowledge of 20th century product design history and designers wouldn’t hurt either.
ID is not about designing what you feel is appropriate (i.e. expressing your creativity) but rather designing useful products that improve someones quality of life and ultimately sell or make money. I’m sure I’ll start some flame war with that last comment, but oh well - its the truth.